1. Field of Invention
The present invention relates to monitoring the position of a cable being towed in a body of water.
2. Description of Prior Art
In marine seismic exploration, a seismic cable often as long as two miles is towed behind a vessel. Hydrophones in the cable sense the response to submerged earthen formations to seismic energy. It is desirable to know as accurately as possible the relative position of the cable along its length with respect to the vessel. U.S. Pats. Nos. 4,231,111; 3,953,827; 4,068,208 and 4,323,990 relate to systems using magnetic compass readings for providing information as to the relative location of a seismic cable behind a vessel. However, so far as is known, these magnetic compasses were mounted within the seismic cables. With such systems repair or replacement of the compasses required disassembly of the cable, which was undesirable, particularly during survey operations. It was also difficult to mount the compasses in a proper position of alignment within the cable.
U.S. Pat. No. 4,267,585 relates to a collar through which a cable was towed. Instrumentation in the collar sensed the relative position of the cable as it passed through the collar to gain an indication of the configuration or profile of the cable in the water.
Another type of electromagnetic compass used in cables was based on an optically coded compass disk connected to a magnet. As the magnet moved due to changes in the earth's magnetic field, the disk moved correspondingly. The surface of the coded disk was scanned photoelectrically to provide output signals indicative of the magnet position and thus the earth's magnetic field. Such a compass is shown, for example in U.S. Pats. Nos. 4,117,602; 4,109,389; 4,032,222; 4,027,398; 3,888,016; 3,772,503; 3,833,901; 3,832,787; 3,927,474 and 4,047,168.